This proposal describes an experimental program for determining the molecular mechanism(s) by which proteins are precipitated from aqueous solution by the addition of water-soluble synthetic polymers, in particular, poly(ethylene glycol) (PEG). The approach involves a study of the molecular structure and solution properties of PEG, coupled with an extensive investigation of its effect on the solubility of proteins. Special attention will be given to the role of protein-protein interactions in determining the response to PEG. Proteins whose reversible self-association has been well characterized will be used as models to evaluate the feasibility of controlling the precipitation of particular proteins through selective manipulation of their oligomeric state. Sedimentation, gel filtration and fluorescence measurements will be used to determine the extent to which PEG promotes or otherwise interferes with protein-protein interactions at concentrations of PEG below those required for precipitation. Optical and radioactive labels will be used to investigate mixtures in order to determine how the presence of additional proteins affects the precipitation of a given protein. The conformational properties of PEG in solution and its interaction with proteins will be investigated using fluorescent probes attached to the termini of the polymer. As knowledge of the mechanism of precipitation emerges, increasing attention will be given to the characterization of selected plasma proteins with respect to those factors found to govern the response to PEG. This basic knowledge will facilitate the development of improved plasma fractionation schemes by enabling the PEG-precipitation curves to be selectively manipulated for particular applications.